Optical head and drive device for optical recording medium

ABSTRACT

An optical head ( 1 ) having an optical means for focusing a laser beam to irradiate an optical recording medium ( 51 ) is equipped with an objective lens ( 2 ) focusing a laser beam L, and a solid immersion lens ( 3 ) disposed between the objective lens ( 2 ) and the optical recording medium ( 51 ), the head comprising a holder member ( 4 ) for integrally retaining the lenses ( 2 ) and ( 3 ) and a moving mechanism ( 5   a ) for moving the holder member in a direction along an optical axis of the lenses ( 2 ) and ( 3 ), wherein an electrically conductive material is used on a surface of the optical head facing the optical recording medium ( 51 ). The numerical aperture can be made large by the solid immersion lens ( 3 ). Moreover, the air gap can be controlled with high precision according to the electrostatic capacity formed between the electrically conductive material and the optical recording medium ( 51 ). Furthermore, the moving mechanism ( 5   a ) and the signal processing for the focus servo operation can be made simpler.

TECHNICAL FIELD

The present invention belongs to a technical field for recordinginformation and reproducing it using an optical recording medium.Particularly, the invention relates to what is suitably used forachieving high-density recording by increasing the numerical aperture ofan objective lens for focusing a laser beam.

BACKGROUND ART

In recent years, there has been a demand for increasing the recordingdensity of a rewritable disc such as a phase-change type optical disc ora magneto-optical disc in order to enable recording a large amount ofdata such as moving-video data without increasing the diameter of thedisc. This increase in recording density is realized by making small ofthe spot size of a laser beam irradiating a signal-recording surface ofthe optical disc.

It is known that the spot size (d) in the order near the wavelength λ ofthe laser beam, according to the Fourier image formation theory, can bedetermined from the wavelength λ and the numerical aperture NA of theobjective lens for focusing the laser beam by the following equation(1).

d=1.22·λ/NA  (1)

Accordingly, the shorter the wavelength of the laser beam is and thegreater the numerical aperture of the objective lens is, the smaller thespot size becomes. This makes it possible to increase the recordingdensity.

As a method of increasing the numerical aperture, there is known amethod using a solid immersion lens.

Stating in terms of its principle, as illustrated in FIG. 1A, thismethod disposes, between an objective lens 61 and an optical disc 62, asolid immersion lens (SIL) 63 whose surfaces facing the objective lens61 and optical disc 62 are a spherical surface 63 a and a flat surface63 b, respectively and causes a laser beam L having passed through theobjective lens 61 to vertically enter the spherical surface 63 a of theSIL 63 and to focus on a central portion of the flat surface 63 b.Assuming that n represents the refractive index of the SIL 63, thenumerical aperture (effective numerical aperture) of a lens groupconsisting of the objective lens 61 and the SIL 63 becomes n times asgreat as the numerical aperture of the objective lens 61 itself.

However, the method is actually arranged to cause the effectivenumerical aperture to become as n² times as great as that of theobjective lens 61 by satisfying the conditions of stigmatic focusing asdescribed below. To this end, as illustrated in FIG. 1B, the methodcauses the laser beam L that has passed through the objective lens 61 toenter the spherical surface 63 a of the SIL 63 at an incidence angledifferent from the angle vertical to this spherical surface 63 a,thereby causing the laser beam L to be somewhat refracted by thespherical surface 63 a.

In, for example, S. M. Mansfield et al's thesis entitled“High-numerical-aperture lens system for optical storage”, carried onpages 305 to 307 of “Optics Letter” the 18th issue, published in 1993(hereinafter called “reference literature no. 1”), H. J. Mamin et al'sthesis entitled “Near-field optical data storage”, carried on pages 141to 143 of “Applied Physics Letter” the 68th issue, published in 1996(hereinafter called “reference literature no. 2”), it is reported thatby the method using the solid immersion lens, the numerical apertureexceeding 1 is realized.

By the way, when the numerical aperture exceeds 1 in that way, as thedistance (air gap) between the solid immersion lens and the optical discin a direction along an optical axis of the laser beam increases, thereflectance of a component of the laser beam which corresponds to theoptical quantity exceeding 1 of numerical aperture at a flat surface ofthe solid immersion lens increases. As a result of this phenomenon, theintensity of the laser beam permeating the solid immersion lens andirradiating the optical disc rapidly deteriorates. When this air gapbecomes more than the range of the proximity field (nearfield), most ofthe component corresponding to the optical quantity exceeding 1 ofnumerical aperture becomes reflected by the flat surface of the solidimmersion lens. Therefore, the intensity of the laser beam irradiatingthe optical disc becomes remarkably low.

In order to represent this specifically, FIG. 2 is made regarding eachcase for the air gap of 0 nm, 50 nm, 100 nm, 200 nm and 500 nm asfollows. The distance on the signal-recording surface of the opticaldisc as measured from the center of the spot of the laser beam isplotted on the abscissa axis. In contrast, the intensity of the laserbeam irradiating this signal-recording surface (as expressed in terms ofthe ratio to the intensity of the laser beam at the spot center thereofwhen the air gap is 0 nm) is plotted on the ordinate axis. FIG. 2 showsthe calculated values of the intensity distribution (Strehl intensity)of the laser beam on the signal-recording surface when the numericalaperture NA=1.5 and the wavelength λ=640 nm.

FIG. 2 shows the following. When the air gap is 50 nm, the intensity ofthe laser beam at the center of the spot is to an extent of 85% of theintensity exhibited when the air gap is 0 nm. However, when the air gapincreases up to 100 nm, the resulting intensity becomes approximately60% of the intensity when the air gap is 0 nm. When the air gap reaches200 nm, the resulting intensity falls down to approximately 35% of theintensity when the air gap is 0 nm.

Therefore, where the numerical aperture exceeds 1, control should beperformed so as to make the air gap sufficiently small (in the case ofthe example illustrated in FIG. 2, so as to keep the air gap fall withinthe range of 100 nm or less at the maximum, or preferably 50 nm or so).Otherwise, due to the fall in the intensity of the laser beamirradiating the signal-recording surface of the optical disc, therecording precision and the reproduction precision willdisadvantageously deteriorate.

As the method of controlling so as to make the air gap small, there isalso a method in which the optical head having an objective lens andsolid immersion lens installed therein is floated from the optical discby the air current involved by the rotation thereof as in the case ofthe magnetic head of a hard-disc device.

However, by this method, the intensity of the air current depends on thelinear velocity of the optical disc, so that, for example, in the caseof a CAV (constant-angular-velocity recording) system, as theirradiating position of the laser beam in the radial direction of thedisc changes (i.e., as the track to be accessed changes), the amount offloating of the head inconveniently changes. In the case of a CLV(constant-linear-velocity recording) system also, the amount of floatingdiffers between optical disc devices whose linear velocities differ fromeach other. As a result, this method is difficult to control the air gapwith high precision.

In view of the above, the applicant of this patent application alreadyfiled in Japanese Patent Office (Japanese Patent Application Laid-OpenNo. 8-212579) a patent application for an invention concerning anoptical head and a driving apparatus for an optical recording medium.This previous invention is arranged to retain an objective lens and asolid immersion lens by respective separate holders and use electricallyconductive material in the holder for retaining the solid immersionlens, thereby causing the control of the position in the direction alongan optical axis of the solid immersion lens in accordance with theelectrostatic capacity (capacitor) formed with this electricallyconductive material to be performed independently of the control of thedistance in the direction along the optical axis between the objectivelens and the optical disc. According to this previous invention, it ispossible to control the air gap with high precision regardless of thelinear velocity of the optical disc.

However, in this previous invention proposed by the present applicant,two actuators are necessary as an actuator for moving the lens in theoptical-axial direction for the purpose of a focus servo. One is anactuator for moving the holder having the solid immersion lens retainedthereby and the other is an actuator for moving the holder having theobjective lens retained thereby.

Also, as the signal processing for producing a control signal for focusservo, two kinds of signal processing are necessary. One is the signalprocessing for producing a control signal that is intended to controlthe position of the solid immersion lens in accordance with theelectrostatic capacity. The other is the signal processing for producinga control signal intended to control the distance between the objectivelens and the optical disc (e.g. the matrix processing of the outputsignal of a photo-detector having received a laser beam reflected fromthe optical disc).

Therefore, the present invention has an object to provide an opticalhead and a driving apparatus for an optical recording medium, which arecapable of making the numerical aperture greater with a solid immersionlens, controlling the air gap according to the electrostatic capacitywith high precision, and also making simpler of the actuator and thesignal processing for focus servo.

DISCLOSURE OF THE INVENTION

An optical head according to the present invention, as described inclaim 1, is characterized by comprising a holder member for retaining anoptical means having the function of an objective lens focusing a laserbeam to irradiate an optical recording medium and an optical meanshaving the function of a solid immersion lens disposed between theobjective lens and the optical recording medium while keeping a fixeddistance between the objective lens and the solid immersion lens, and amoving mechanism for moving the holder member in a direction along anoptical axis of the laser beam, wherein an electrically conductivemember is used on a surface of the optical head facing the opticalrecording medium.

In this optical head, the optical means having the function of anobjective lens and optical means having the function of a solidimmersion lens are integrally retained by one holder member with thedistance between the objective lens and the solid immersion lens beingkept fixed. This holder member is moved in the optical-axial directionof the laser beam by one moving mechanism. Therefore, the objective lensand the solid immersion lens are moved together in the optical-axialdirection by the one moving mechanism as the distance therebetween iskept fixed.

Then, since an electrically conductive material is used on the surfacefacing the optical recording medium, an electrostatic capacity is formedbetween this electrically conductive material and the optical recordingmedium.

Accordingly, in a focus servo system of the driving apparatus for theoptical recording medium, when producing a control signal forcontrolling the distance (air gap) between the solid immersion lens andthe optical recording medium as measured in the optical-axial direction,according to the electrostatic capacity, and moving this holder memberaccording to this control signal, with this moving mechanism, theobjective lens and the solid immersion lens are simultaneously moved inthe optical-axial direction. Consequently, the air gap is controlledwith high precision and simultaneously the distance between theobjective lens and the optical disc is also controlled with highprecision. Thus, the focus servo is realized.

In this way, according to the above optical head, the numerical aperturecan be made great (e.g. more than 1) by the solid immersion lens. Inaddition, the control for making the air gap sufficiently small (e.g.within 100 nm) can be performed with high accuracy according to theelectrostatic capacity. Furthermore, since focus servo can be realizedwith a single moving mechanism (actuator), it is possible to makesimpler of the moving mechanism for performing focus servo.

Also, on the side of the focus servo system of the driving apparatus foran optical recording medium, it is possible to realize focus servo witha sort of signal processing based on the electrostatic capacity. Thisenables to make simpler of the signal processing for the purpose ofexecuting the focus servo.

It is noted that, in this optical head, as stated in claim 2, if asurface facing to the optical recording medium of the solid immersionlens is provided with a protrusion at its central portion, a peripheralportion around the protrusion being made flat, and on this peripheralportion being formed a film made of an electrically conductive material,then an electrostatic capacity is suitably formed between the solidimmersion lens itself and the optical recording medium.

As the result, for example, compared to the case where forming theelectrostatic capacity, e.g. between the holder member and the opticalrecording medium, the value of the electrostatic capacity can beincreased by making small of the distance between the electricallyconductive material and the optical recording medium. Therefore, it willbe possible to produce the control signal based on the electrostaticcapacity with higher precision.

Also, as illustrated in FIG. 1 as well, because the laser beam passesthrough the central portion of the surface of the solid immersion lensfacing the optical recording medium, the film formed on the peripheralportion of this facing surface will be no obstacle for the laser beam toirradiate the optical recording medium.

Moreover, corresponding to the protrusion of the facing surface at thecentral portion, the remaining portion of the optical head is retreatedaway from the optical recording medium. Therefore, even when the entireoptical head has been inclined with respect to the optical recordingmedium, it is less likely that the optical head will contact with theoptical recording medium.

Furthermore, in this optical head, as stated in claim 3, it is morepreferable that an electrically conductive material is used for theholder member in order to electrically connect the holder member and theelectrically conductive film of the peripheral portion of the solidimmersion lens.

This makes it possible to detect the value of the electrostatic capacitythrough the holder member, thus allowing the value of the electrostaticcapacity to be easily detected.

Next, the driving apparatus for the optical recording medium accordingto the present invention is equipped, as stated in claim 5, with thefollowing optical head. The optical head includes a holder member forretaining an optical means having the function of an objective lensfocusing a laser beam to irradiate an optical recording medium and anoptical means having the function of a solid immersion lens disposedbetween the objective lens and the optical recording medium whilekeeping fixed the distance between the objective lens and the solidimmersion lens, and a moving mechanism for moving the holder member in adirection along an optical axis of the laser beam, wherein anelectrically conductive material is used on a surface of the opticalhead facing the optical recording medium. At the same time, theapparatus is also equipped with a signal processing means that producesa control signal for controlling the distance between the solidimmersion lens and the optical recording medium as measured in theoptical-axial direction, according to the electrostatic capacity formedby this electrically conductive material and this optical recordingmedium, wherein the holder member is moved by means of the movingmechanism according to the control signal.

In this driving apparatus for an optical recording medium there areprovided not only the optical head as stated in claim 1, but also thesignal processing means for producing a control signal that controls thedistance between the solid immersion lens and the optical recordingmedium according to the electrostatic capacity formed in that opticalhead. By moving the holder member with the moving mechanism according tothe control signal, the air gap is controlled with high accuracy andsimultaneously the objective lens to optical disc distance is alsocontrolled with high accuracy. Therefore, the focus servo is realized.

In this way, according to the driving apparatus for an optical recordingmedium, it is possible to make the numerical aperture large by means ofthe solid immersion lens. In addition, it is possible to make controlwith high precision for making the air gap sufficiently small accordingto the electrostatic capacity. Furthermore, it is possible to realizethe focus servo with a single moving mechanism and a sort of signalprocessing based on the electrostatic capacity. Thus, the movingmechanism and signal processing for use in the focus servo can be madeto be simpler.

Additionally, this signal processing means of the driving apparatus foran optical recording medium may be arranged, for example, as stated inclaim 6, to have a means for generating a signal, either of thefrequency or the phase of which changes depending on a change in theelectrostatic capacity, a means for generating a predetermined referencesignal, and a means for producing a control signal by comparing eitherof the frequency or phase of the above signal with that of the referencesignal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are diagrams illustrating the principle according towhich the numerical aperture is increased with a SIL.

FIG. 2 is a graphic diagram illustrating the intensity distribution of alaser beam on an optical disc when the numerical aperture exceeds 1.

FIG. 3 is a side view, partly in section, of an example of the structureof an optical head according to the present invention.

FIG. 4 is a side view illustrating a detailed example of the structureof a bottom surface of the SIL of FIG. 3.

FIG. 5 is a block diagram illustrating an example of the construction ofa servo signal processing system of a driving apparatus for an opticaldisc according to the present invention.

FIG. 6 is a diagram illustrating an example of the structure of anoptical pick-up of FIG. 5. And,

FIG. 7 is a diagram illustrating an example of the disposition of lightreceiving elements on a photo-detector 38 of FIG. 6.

BEST MODE FOR CARRYING OUT THE INVENTION

In the following description, an explanation will be given of an examplein which the present invention is applied to a driving apparatus for aphase-change type optical disc.

FIG. 3 illustrates an example of the construction of an optical headfixed in an optical pick-up of this optical-disc-driving apparatus.

This optical head 1 comprises an objective lens 2 for focusing a laserbeam L to irradiate a phase-change type optical disc 51 mounted on thedisc-driving apparatus, a solid immersion lens (SIL) 3 that is disposedbetween the optical disc 51 and the objective lens 2, a lens holder 4that integrally holds these lenses 2 and 3, and an electromagneticactuator 5 for moving this lens holder 4 (a focus actuator 5 a formoving in the optical-axial direction of the lenses 2 and 3 and atracking actuator 5 b for moving in a direction along the disc surfaceof the optical disc 51).

In this way, the objective lens 2 and the SIL 3 are integrally held byone lens holder 4, and this lens holder 4 is moved by one focus actuator5 a in the direction along the optical axis. It is thus arranged thatthe objective lens 2 and the SIL 3 are moved by one focus actuator 5 ain the optical-axial direction as the distance therebetween is keptfixed.

The SIL 3 has a shape in which a part of a spherical lens is cut away(It is generally called “Super Sphere SIL” or “Hyper Sphere SIL”). Inthis SIL 3, the spherical surface is made to face the objective lens 2and the surface opposite to the spherical surface (hereinafter called “abottom surface”) is made to face the optical disc 51. In this conditionthe SIL 3 is held by the lens holder 4.

This SIL 3 is designed so as to focus the laser beam L withoutaberration (to satisfy the stigmatic focusing conditions). Assuming thatr represents the radius of the spherical lens and n represents therefractive index of the spherical surface, the thickness t of the SIL 3as measured in the optical-axial direction is determined using thefollowing expression (2).

t=r·(1+1/n)  (2)

In the reference literature no. 2 above referred to, it is reported thatthe numerical aperture of a lens group consisting of the objective lens2 and the SIL 3 (the effective numerical aperture) NA_(eff) is foundfrom the numerical aperture of the objective lens 2 NA_(obj) and therefractive index n by the following expression (3).

NA _(eff) =n ² ·NA _(obj)  (3)

By way of example, a lens having a numerical aperture NA_(obj)=0.45 isherein used as the objective lens 2 and a lens having a refractive indexn=1.83 is used as the SIL 3. Accordingly, from the expression (3) theNA_(eff) becomes approximately 1.5. Therefore, when the wavelength ofthe laser beam L is, for example, 640 nm, for the reason that has beenexplained using FIG. 2, the SIL 3 to optical disc 51 distance (air gap)as measured in the optical-axial direction of the laser beam L needs tobe controlled within 100 nm even at maximum. preferably to 50 nm or so.

FIG. 4 illustrates a detailed example of the structure of the bottomsurface of the SIL 3 of FIG. 3. This bottom surface has a diameter D of1.5 mm and its central portion 3 a is caused to protrude. Its peripheralportion 3 b is made flat. The width φ and the height of the centralportion 3 a are about 40 μm and 2 μm, respectively.

On the peripheral portion 3 b there is formed a film 6 made ofelectrically conductive material (by way of example, this is made to bealuminum) so as to be sufficiently thinner than the height of theprotrusion of the central portion 3 a. As a result of this structure, anelectrostatic capacity is formed between the aluminum film 6 and thealuminum-made reflection plane of the optical disc 51.

The value C of this electrostatic capacity is found, on the assumptionthat S represents an area of a surface where the peripheral portion 3 band the optical disc 51 are opposite to each other and h represents thedistance between the aluminum film 6 and the optical disc 51, from thefollowing expression (4).

C=∈0·∈r·S/h  (4)

where ∈0 represents the vacuum dielectric constant (8.854×10−₁₂ (F/m);and ∈r represents the relative dielectric constant (approximately 1within the air).

Since the diameter D of the bottom surface of the SIL 3 is 1.5 mm asmentioned above, the area S becomes approximately 1.766×10⁻⁶m². When thecentral portion 3 a is in contact with the optical disc 51 (when the airgap is 0), the distance h has a minimum value of approximately 2 μm.When the air gap is 50 nm, 100 nm, and 200 nm, the distance h has valuesof about 2.05 μm, 2.1 μm, and 2.2 μm, respectively.

Accordingly, when the air gap is 0, 50 nm, 100 nm and 200 nm, the valuesC of the resulting electrostatic capacities are found from theexpression (4) to be 7.82 pF, 7.63 pF, 7.45 pF, and 7.11 pF,respectively.

As illustrated in FIG. 3, this aluminum film 6 is joined to the lensholder 4 by a solder 7. The lens holder 4 is made of an electricallyconductive material (It is herein, e.g. aluminum). It is thus arrangedthat a voltage signal indicating the value C of the electrostaticcapacity can be taken out from the lens holder 4 (i.e, the value C ofthe electrostatic capacity can be detected through the lens holder 4).

Next, FIG. 5 illustrates an example of the construction of a focus servoand tracking servo signal processing system of an optical disc drivingapparatus in which the optical head 1 of FIG. 3 is fixed in an opticalpick-up.

The optical disc 51 mounted on this optical disc driving apparatus isdriven to rotate by a spindle motor 11 on the CAV(constant-angular-velocity recording) system.

By a laser beam with a wavelength of 640 nm being made to irradiate thisoptical disc 51 through the operations of the optical pick-up 12 and theoptical head 1 as described below information is recorded onto andreproduced from the optical disc 51.

The signal processing system for use in the focus servo operation isconstructed as follows.

The voltage signal indicating the value C of the electrostatic capacity,which has been taken out from the lens holder 4 (FIG. 3) of the opticalhead 1, is supplied to a VCO (voltage-controlled oscillator) 13.

The VCO 13 consists of a LC oscillator and outputs a signal having anoscillation frequency f as is given in the following expression (5)according to the value C of the electrostatic capacity indicated by thevoltage signal and the inductance L having a fixed value inside the VCO13.

 f=½π{square root over (LC)}  (5)

where, by way of example, it is assumed that the inductance L inside theVCO 13 is equal to 100 μH.

When the air gap is 0, 50 nm, 100 nm, and 200 nm, the values C of theelectrostatic capacities respectively become 7.82 pF, 7.63 pF, 7.45 pFand 7.11 pF from the expression (4), respectively. Accordingly, thecorresponding oscillation frequencies f of the VCO 13 become 5.69 MHz,5.76 MHz 5.83 MHz and 5.97 MHz from the expression (5), respectively.

The output signal of the VCO 13, along with a signal having a referencefrequency of 5.76 MHz (namely, the frequency equal to the oscillationfrequency f of the VCO 13 output when the air gap is 50 nm), which isoutput from a VCXO (voltage-controlled oscillator) 14, is supplied to aPLL (phase-locked loop) 15 serving as a frequency/phase comparator.

The PLL 15 compares the frequency and phase of the output signal of theVCO 13 and those of the output signal of the VCXO 14 and outputs asignal corresponding to the error between the output both signals withrespect to the frequency and phase.

The output signal of the PLL 15 is phase-compensated by a phasecompensator circuit 16 and then is amplified by an amplifier 17.Thereafter, the resulting signal is supplied to the focus actuator 5 aof the electromagnetic actuator 5 of the optical head 1 as a controlsignal for controlling the air gap.

The focus actuator 5 a moves the lens holder 4 in the optical-axialdirection according to this control signal. This makes the air gapcontrolled to be 50 nm and also the distance between the objective lens3 and the optical disc 51 is also controlled to be fixed. Thus, thefocus servo is realized.

On the other hand, the signal processing system used for the trackingservo operation is constructed as follows.

FIG. 6 illustrates an example of the structure of the optical pick-up12. A linearly polarized laser beam L having a wavelength of 640 nm,which is emitted from a semiconductor laser 31, is made to be a parallellight by a collimator lens 32. The parallel light is separated by adiffraction grating 33 into a main beam (the 0th order light) and sidebeams (the ±1st order lights) and its plane of polarization is rotatedby a ½-wavelength plate 34. Then, the resulting beam enters apolarized-beam splitter 35.

Most of this incoming beam passes through the polarized-beam splitter 35and is made to be a circularly polarized light by a ¼-wavelength plate36. The resulting beam is focused by the objective lens 2 and the SIL 3(FIG. 3) of the optical head 1 and irradiates a signal recording surfaceof the optical disc 51.

Additionally, part of the incoming beam is reflected by thepolarized-beam splitter 35 and then passes through a condenser lens 39enter a photo-detector 40 for monitoring the intensity of the laserbeam.

The laser beam reflected by the signal-recording surface of the opticaldisc 51 passes through the optical head 1 and is made to be a linearlypolarized light orthogonal to the original one by the ¼-wavelength plate36. The resulting light is reflected by the polarized-beam splitter 35and the reflected light passes through a condenser lens 37 to enter aphoto-detector 38 for detecting a tracking error signal and a RF signal.

As illustrated in FIG. 7, in the photo-detector 38, there are disposedfour-divided light-receiving elements (photo-diodes) 38A to 38D forreceiving the main beam at a central portion thereof. On both sidesthereof, there are disposed two-divided light-receiving elements 38E,38F and 38G, 38H for receiving the sidebeams. Thus, the photo-detector38 is a photo-detector that is divided into eight elements.

The output signals A to H of the respective light-receiving elements 38Ato 38H of the photo-detector 38 are amplified by a head amplifier 18 andthen supplied to a tracking matrix circuit 19 as illustrated in FIG. 5.

The tracking matrix circuit 19 generates a tracking error signal TE byperforming the calculation of the following expression (6) according tothe output signals A to H.

TE=(A+D)−(B+C)+k·{(E−F)+(G−H)}  (6)

where k represents a constant.

Additionally, the output signals A to D of the respectivelight-receiving elements 38A to 38D of the photo-detector 38 areamplified by a head amplifier 18, and then also supplied to thereproduction signal processing system (not illustrated) of this opticaldisc driving apparatus. In this reproduction signal processing system, areproduction RF signal RF is generated by performing the calculation ofthe following expression (7).

RF=A+B+C+D  (7)

The tracking error signal TE is phase-compensated by a phase compensator20, amplified by an amplifier 21, and then supplied to the trackingactuator 5 b of the electromagnetic actuator 5 of the optical head 1.

The tracking actuator 5 b moves the lens holder 4 according to thetracking error signal TE in a direction along the disc surface of theoptical disc 51, thus the tracking servo control being realized.

These focus servo and tracking servo control operations are performedunder the control of a CPU 22.

Moreover, the output signal of the photo-detector 40 (FIG. 6) formonitoring the optical pick-up 12 is supplied to an APC (automatic powercontrol) circuit 23, which adjusts, under the control of the CPU 22, theoutput level of the semiconductor laser 31 (FIG. 6) of the opticalpick-up 12 so that the value of the output signal of the photo-detector40 may coincide with a predetermined reference value (there are tworeference values therefor, one being for the recording mode and theother being for the reproduction mode).

As described above, according to this optical disc driving apparatus, asa result of using the solid immersion lens, the numerical aperture ismade to be approximately 1.5 and besides, control of the air gap to 50nm (size allowing no fall in the intensity of the laser beam irradiatingthe signal-recording surface of the optical disc 51) is performedaccording to the electrostatic capacity with high precision.

Furthermore, according to this optical disc driving apparatus, becausethe focus servo is realized with a single focus actuator 5 a (FIG. 3),compared to the case where two focus actuators are needed, one being formoving the holder retaining the solid immersion lens and the other beingfor moving the holder retaining the objective lens, the focus actuatoris further simplified.

Also, because the focus servo is realized through a single signalprocessing system from the VCO 13 based on the electrostatic capacity toamplifier 17 (FIG. 5), compared to the case where two signal processing,one being systems are required for the focus servo, for generating acontrol signal for controlling the air gap according to theelectrostatic capacity and the other being for generating a controlsignal for controlling the distance between the objective lens and theoptical disc (e.g. the matrix processing of the output signal of thephoto-detector which receives the laser beam reflected by the opticaldisc), the signal processing for the focus servo is further simplified.

Moreover, as illustrated in FIG. 4, because there is formed theelectrostatic capacity between the SIL 3 itself and the optical disc 51by forming the aluminum film 6 on the bottom surface of the SIL 3,compared to the case where the electrostatic capacity is formed, forexample, between the lens holder 4 and the optical disc 51, the value Cof the electrostatic capacity is made larger due to the reduction in thedistance between the aluminum film 6 and the optical disc 51. Therefore,it is possible to produce the control signal for the focus servoaccording to the electrostatic capacity with higher precision.

Furthermore, because the central portion 3 a of the bottom surface ofthe SIL 3 is made into the protrusion and, on the peripheral portion 3 bthereof, the aluminum film 6 is formed thinner than the height of thisprotrusion, when the air gap is controlled using the central portion 3 aas a reference, there is no possibility that the aluminum film 6 willapproach the optical disc 51 closer than o the central portion 3 a doesto contact with the optical disc 51.

In addition, because the central portion 3 a of the bottom surface ofthe SIL 3 protrudes, the remaining portion of the optical head 1 isretreated from the optical disc 51 accordingly. Therefore, even when theentire optical head 1 is inclined with respect to the optical disc 51,there is less possibility that the optical head 1 will contact with theoptical disc 51.

Also, as illustrated in FIG. 3, because it is arranged that the value Cof the electrostatic capacity can be detected through the lens holder 4by electrically connecting the aluminum film 6 to the aluminum-made lensholder 4, the value C of the electrostatic capacity can easily bedetected.

Additionally, in the above example, the optical head 1 is provided withtwo lenses of the objective lens 2 and the SIL 3 as the optical meanshaving the function of an objective lens for focusing the laser beam toirradiate the optical disc and the optical means having the function ofa solid immersion lens disposed between the objective lens and theoptical disc.

However, the present invention is not limited there to. For instance, itmay be arranged that the optical head is provided with a single opticalelement that has both of the function of an objective lens and thefunction of a solid immersion lens.

As such a single optical element, for example, there can be cited areflection type light condensing element that is described in the ChulWoo Lee et al's thesis entitled “Feasibility study on near field opticalmemory using a catadioptric optical system”, carried on pages 137 to 139of “Digest of Optical Data Storage” published by Aspen company in 1998.

Alternatively, as the optical means having the function of an objectivelens and the optical means having the function of a solid immersionlens, three or more optical elements may be provided in the optical heador a hologram element may be provided in the optical head.

Also, in the above example, the present invention is applied to thedriving apparatus for a phase-change type optical disc. However, thepresent invention may be also applied to a driving apparatus for amagneto-optical disc, a driving apparatus for an optical disc dedicatedto reproduction only, or a driving apparatus for an optical recordingmedium (e.g. an optical card) other than the optical disc.

The present invention is not limited to the above-described example andof course permits other various constructions can be made withoutdeparting from the scope of the invention.

As having described above, according to the optical head of the presentinvention, the numerical aperture can be made large (e.g. more than 1)by means of the solid immersion lens and besides, the control for makingthe air gap sufficiently small (e.g. within 100 nm) can be performedaccording to the electrostatic capacity with high precision.Furthermore, focus servo can be realized with a single moving mechanism(actuator). Therefore, it is possible to further simplify the movingmechanism for performing focus servo.

Moreover, on the side of the focus servo system of the driving apparatusfor an optical recording medium, it is also possible to realize focusservo with a single signal processing system based on the electrostaticcapacity. This advantageously makes it also possible to further simplifythe signal processing for the purpose of executing the focus servo.

In addition, the central portion of a surface of the solid immersionlens facing the optical recording medium is made into a protrusion aperipheral portion around the protrusion being made flat, and on thisperipheral portion being formed a film made of an electricallyconductive material to form an electrostatic capacity between the solidimmersion lens itself and the optical recording medium. When arranged inthis manner, compared to the case, e.g. where the electrostatic capacityis formed between the holder member and the optical recording medium,the value of the electrostatic capacity can be made large by makingsmall of the distance between the electrically conductive material andthe optical recording medium. Therefore, it advantageously becomespossible to produce the control signal based on the electrostaticcapacity with higher precision.

Also, by the extent to which the central portion of this facing surfaceprotrudes, the remaining portion of the optical head is retreated awayfrom the optical recording medium. Therefore, even when the entireoptical head has been inclined with respect to the optical recordingmedium, it advantageously becomes less possible that the optical headwill contact with the optical recording medium.

Furthermore, if it is arranged to electrically connect the holder memberusing an electrically conductive material and the electricallyconductive film of the peripheral portion of the solid immersion lens,it is then possible to detect the value of the electrostatic capacitythrough the holder member. Therefore, it advantageously becomes possibleto easily detect the value of the electrostatic capacity.

Next, according to the driving apparatus for an optical recording mediumby the present invention, it is possible to make the numerical aperturelarge with the solid immersion lens and besides, to perform a controlfor making the air gap sufficiently small according to the electrostaticcapacity with high accuracy. Furthermore, it is possible to realize thefocus servo with a single moving mechanism and a single signalprocessing system based on the electrostatic capacity. Therefore, it isadvantageously possible to further simplify the moving mechanism andsignal processing for the focus servo.

What is claimed is:
 1. An optical head comprising a holder member forretaining an optical means having the function of an objective lensfocusing a laser beam to irradiate an optical recording medium and anoptical means having the function of a solid immersion lens disposedbetween the objective lens and the optical recording medium whilekeeping a fixed distance between the objective lens and the solidimmersion lens, and a moving mechanism for moving the holder member in adirection along an optical axis of the laser beam, wherein anelectrically conductive material is used on a surface of the opticalhead facing the optical recording medium.
 2. The optical head accordingto claim 1, wherein a surface of the solid immersion lens facing theoptical recording medium is caused to protrude at its central portionand its peripheral portion being made flat, and a film made of anelectrically conductive base material being formed on the peripheralportion.
 3. The optical head according to claim 2, wherein anelectrically conductive material is used for the holder member, and thefilm formed on the peripheral portion is electrically connected to theholder member.
 4. The optical head according to any one of claims 1 to3, wherein the numerical aperture of a lens group consisting of theobjective lens and the solid immersion lens exceeds
 1. 5. A drivingapparatus for an optical recording medium, for driving the opticalrecording medium to perform either of the recording of information ontothe optical recording medium and the reproduction of information fromthe optical recording medium, comprising an optical head including aholder member for retaining an optical means having the function of anobjective lens focusing a laser beam to irradiate the optical recordingmedium and an optical means having the function of a solid immersionlens disposed between the objective lens and the optical recordingmedium while keeping a fixed distance between the objective lens and thesolid immersion lens as well as a moving mechanism for moving the holdermember in a direction along an optical axis of the laser beam, in whichan electrically conductive material is used on a surface of the opticalhead facing to the optical recording medium, and signal processing meansthat produces, according to the electrostatic capacity formed by theelectrically conductive material and the optical recording medium, acontrol signal for controlling the distance between the solid immersionlens and the optical recording medium as measured in the optical-axialdirection, wherein the holder member is moved by the moving mechanismaccording to the control signal.
 6. The driving apparatus for an opticalrecording medium according to claim 5, wherein the signal processingmeans includes a means for generating a signal whose frequency or phasechanges in accordance with a change of the electrostatic capacity, ameans for generating a predetermined reference signal, and a means forproducing the control signal by comparing either of the frequency andphase of the signal with that of the reference signal.
 7. The drivingapparatus for an optical recording medium according to claim 5 or 6,wherein the numerical aperture of a lens group consisting of theobjective lens and the solid immersion lens exceeds 1, and the signalprocessing means produces a signal for controlling the distance to fallwithin 100 nm.